Temporary fix for obscure scrolling bug.
Temporary fix for obscure scrolling bug.

I'm seeing a strange scrolling bug on some pages, for example, here:
http://www.steephill.tv/tour-of-qatar/.

Background:
   When we scroll, we first have to decide which element to scroll.
   Because of a chrome bug, we do this by testing candidate elements:
   scroll the element up by one px, then back down again.  If it moved,
   then it's scrollable, and it's the element we should scroll.

Bug:
   In some cases, the micro-scroll-up succeeds, but the subsequent
   micro-scroll-down fails.  It's not clear how this could happen.  It
   may be a chrome bug.  In any case, as a result, we conclude that the
   element is not scrollable, and proceed to the next candidate.  We end
   up finding no scrollable element, and j/k scrolling is broken.  If
   you press-and-hold "j" or "k", you can see the repeated
   micro-scroll-ups.

Problem:
   I can't reproduce this in a clean chrome account.  I only see it in
   my live chrome instance.  I've tried disabling all extensions, but
   the problem persists.  So there's something else at play, but I
   haven't yet been able to track it down.

Solution:
   This commit is a temporary workaround.  We just default to scrolling
   document.body if we can't find anything else to scroll.  Most of the
   time, that's the right thing to do.  And it's what we would have done
   prior to implementing smooth scrolling.  Nevertheless, it isn't a
   great solution...

Notes:
   It's possible that this is related to #1117.  However, users there
   have reported *not* seeing the micro-scrolls.  So probably not.

   I'm not posting this as a PR, because I can't reliably reproduce the
   bug.  However, the fix pretty much does what we did before smooth
   scrolling, it's almost certainly safe.  So I'm just pushing this
   straight into master.

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